FStream: Include Configure.hxx before other headers

In commit 15e90a3c (Sort includes to stabilize include order w.r.t.
clang-format, 2016-11-04) we forgot to move `Configure.hxx` into its own
block to ensure it remains first.  Restore the original include order
and add separating blanks to make it stable w.r.t. clang-format.

Change-Id: Ibca524065147192da916ad98d7db3c5d86b26a5a